Espresso is a thick, dark coffee with a foamy head or crema and is brewed by forcing hot water under pressure through finely ground coffee. The machines for making espresso can be classified into steam, pump, and piston machines depending on how the pressure is generated. In steam machines, the pressure is generated by steam from boiling water in a sealed reservoir. The espresso made by a steam machine is strong and dark but lacks the body and crema of a good espresso, which is caused by the low steam pressure and high temperature of the water used for brewing.
In pump and piston machines the pressure is generated by an electric pump and a hand-operated piston, respectively. Both the electric pump and hand-operated piston can generate a pressure from 10 to 15 bars, which is much higher than the pressure achievable in a steam machine. One can make a good espresso with pump and piston machines, but only after a lot of practice.
The difficulty in making good espresso with the pump and espresso machines is due to the fact that a lot of variables, such as the fineness and compaction of the coffee grounds in the filter holder, may affect the quality of the espresso. For example, too much compaction or too fine grounds is apt to yield a dark espresso with a thin or no foamy head, whereas too little compaction or too coarse grounds yields a pale, weak coffee with little foam.
Numerous filter holders modified for more consistent espresso are known in the art but have not overcome many of the problems associated with such holders. For example, U.S. Pat. No. 4,882,982 to Spidem shows a filter holder that has pressure control means downstream of the coffee filter for keeping consistent high pressure above the grounds. The pressure control means comprises a narrow orifice as a restrictor to the coffee flow to produce a pilot backpressure, and a shut-off valve located upstream of the restrictor and being piloted by the backpressure. A problem arises with the filter holder of Spidem in that the restrictor and shut-off valve are vulnerable to clogging by foreign matter such as coffee grounds. A second problem with Spidem's filter holder is that it requires high pressure to achieve the foamy head and it is not suitable for steam machine. A third problem with the filter holder as shown in Spidem is that it is complicated and difficult to clean.
Another modified filter holder is shown in U.S. Pat. No. 5,127,318 to Selby. The filter holder as shown in Selby also has a pressure control means for controlling the pressure downstream of the coffee grounds. The pressure control means in Selby's filter holder comprises a pressure regulator valve which allows coffee extraction to pass only after a sufficiently high pressure has been reached. The filter holder of Selby resolves the clogging problem but does not provide solutions to the second and third problems of Spidem. In addition, the regulator valve of Selby's holder stops the coffee flow at a high pressure, which may cause residual hot water and pressure in the holder at the end of a brew cycle.
U.S. Pat. No. 5,150,645 to Schiettecatte shows another modified filter holder which incorporates a mechanism to atomize the coffee extraction to improve the foamy head on espresso. The filter holder as shown in Schiettecatte requires very high pressure to obtain any significant degree of atomization and is not suitable for steam machine. As Spidem's filter holder, Schiettecatte's holder stops the coffee flow at a very high pressure, which may cause residual hot water and pressure in the holder at the end of a brew cycle. Furthermore, the atomization of espresso occurring in Schiettecatte's filter holder will cause rapid evaporation or loss of aroma in the espresso, thus leading to an inferior brew.
Cappuccino is made by adding about one-third espresso into two-thirds milk, or vice versa. Prior to mixing milk with espresso, the milk is frothed with a jet of steam to produce a foamy head and to heat the milk to a desired temperature. The simplest frothing device consists of a steam tube having an orifice at one end for producing the steam jet. The espresso machine operators position the orifice just below the surface of milk and bobs the milk container up and down to inject air into the milk. After enough foam is produced, the steam tube is inserted down into the milk to further heat the milk without producing more foam. Here, experience is crucial to obtain satisfactory results because if you have the nozzle too deeply into the milk there will be no foam, if you have it too shallow it will spray milk all over.
A modified form of the frothing device has an air tube with a first end next to the orifice of the steam tube and a second end far above the surface of milk as disclosed in U.S. Pat. No. 4,945,824 by M. Borgmann. Such a modified device can make air available to the milk near the orifice even if the orifice is immersed in the milk. The air tube is very small at its second end to control the flow of air into the tube. One problem arises with this modified frothing device in that after the milk container is full of foam or after the desired amount of foam has been produced, it is difficult to further heat the milk without producing more foam. Consequently, you may have to either satisfy with a lukewarm milk or suffer a mess caused by milk overflowing. A second problem with this device is that the second end of the air tube is very vulnerable to clogging by milk, dusts, etc., thus making the frothing operation inconsistent and frustrating. A third problem arises with this device in that there are various types of milk and they may require more or less air flow to achieve optimum frothing. The air tube in this modified device, however, can not adjust the air flow through it according to the types of milk. U.S. Pat. No. 4,852,474 to Mahlich and Borgmann shows an improved frothing device or attachment having a valving element at the top end of the air tube for adjusting the air flow. The frothing device of Borgmann et al. resolves some of the problems mentioned above but it may require the operator to adjust the valving element during the frothing process.
Another modified frothing device involves a spinning blade at the end of the steam tube. Steam whirls the blade like a miniature turbine to break air bubbles into smaller ones to stabilize the resulting foam. This method, however, requires the operators to keep moving the milk container downward during frothing process to ensure that the air vent in the blade attachment is just at the surface of the milk.
The present invention provides apparatus having novel and improved means for automatically frothing milk and consistently brewing high-quality espresso with a rich foamy head, both in a manner that is independent of the operators' skill, and is aimed at solving the problems associated with the existing espresso/cappuccino machines.